The present invention relates to a method for making printing plates used for lithographic printing which can perform imaging by laser under roomlight, requires no processing solutions, provides images of high resolution, and especially is low in production cost.
A lithographic printing plate comprises an oleophilic image area which is receptive to inks and an hydrophilic non-image area which is not receptive to the inks. The conventional lithographic printing is carried out by feeding both water and ink to the surface of the printing plate to allow the image area to receive preferentially the ink and the non-image area to receive preferentially water, and then transferring the ink on the image area onto a receiver such as paper.
At present, lithographic printing plates are made by providing an oleophilic ink-receptive layer on a substrate such as aluminum sheet, zinc sheet and paper, the surface of which are rendered hydrophilic. In presensitized plates, a photosensitive compound such as diazo compound or photopolymer is applied onto a metallic support the surface of which is rendered hydrophilic. In silver diffusion transfer material, images are formed onto a paper or plastic support by silver diffusion transfer process (DTR process).
According to the method of forming an ink-receptive layer (hereinafter referred to as "image layer") using a diazo compound or photopolymer, first a photosensitive substance such as diazo compound or photopolymer is coated on a substrate such as a metal, paper, laminate, or insulation base. Then, this is exposed to light to allow the photosensitive compound to undergo chemical change, thereby to cause change in solubility of the photosensitive compound in developers. The photosensitive compounds are classified into two depending on the kind of the chemical change. One is negative type in which the area exposed to light is polymerized and cured to become insoluble in developer and another is positive type in which functional group of the area exposed to light changes to have solubility in developer. In both the cases, the image layer comprises the photosensitive compound insoluble in developer which remains on the substrate after processed with developer.
On the other hand, lithographic printing plates made utilizing DTR process, for example, lithographic printing plates having a physical development nuclei layer on a silver halide emulsion layer are disclosed in U.S. Pat. Nos. 3,728,114, 4,134,769, 4,160,670, 4,336,321, 4,501,811, 4,510,228 and 4,621,041. The exposed silver halide crystals are developed chemically by DTR developer to become black, and the exposed area is turned to a hydrophilic non-image area. Unexposed silver halide crystals become a silver salt complex with a silver halide solvent contained in the developer, which diffuses to the surface physical development nuclei layer and undergoes physical development in the presence of the nuclei to form an image area mainly composed of an ink receptive silver. Furthermore, JP-A-63-260491, JP-A-3-116151 and JP-A-4-282295 disclose lithographic printing plates comprising a grained and anodized aluminum base as a support, a physical development nuclei layer and a silver halide emulsion layer coated on the base in succession. These lithographic printing plates are exposed imagewisely and developed in DTR developer, and then the silver halide emulsion layer is washed off with warm water, and the unexposed area is turned to an image area mainly composed of physical development silver on the anodized aluminum base.
In case an image is formed using the above-mentioned photosensitive materials, the method of exposure is one of important factors which determine resolution. Hitherto, in the conventional method presensitized plates are subjected to contact exposure with ultraviolet light or white light through a final film made in the prepress. However, computer-to-plate technology are being developed in which digital data are input, paginated and output to the laser sensitive plate without a final film. The laser direct imaging method has many merits such as low cost, time saving and high productivity for printing production with the trend to a variety of small runs.
In order to employ this laser direct imaging method, the sensitivity of photosensitive materials must be enhanced. In the case of diazo compounds or photopolymers, photochemical reaction is involved and hence they have low sensitivity, namely, several to several hundred mJ/cm.sup.2. Therefore, laser source of high output must be used and there are problems that the apparatuses used are large and the cost is high.
In the silver diffusion transfer technology, the laser sensitive plates are available to the conventional laser diode imagesetter, as its sensitivity is several .mu.J/cm.sup.2, but there are problems that efficiency of production and plate making operation is seriously deteriorated because coating step on the substrate and storage of the materials before exposure must be in the dark or under safelight. Moreover, in the case of diazo compounds and photopolymers, reaction also proceeds under roomlight or sunlight, and, besides, change in reactivity also occurs under high temperatures. Furthermore, if oxygen is present, this serves as an inhibitor for the reaction. Therefore, the materials must also be stored in the dark or in the state of low oxygen content before exposure and development. Furthermore, according to the above methods for the image formation, a processing such as developing solution is generally carried out, and disposal of waste solution is an environmental problem.